This invention generally relates to means for accurately measuring radiation emitted by radionuclides. More specifically, the present invention is directed to an improved liquid scintillation analyzer of the coincident pulse detection type which allows increased accuracy in low level liquid scintillation counting.
Liquid scintillation analyzers have become increasingly common in the accurate measurement of radiation activity in view of their relatively high counting efficiency, even for low energy radioisotopes such as tritium. Although a variety of . techniques are used, in its most basic form liquid scintillation analysis involves dissolving or suspending radionuclides in a solvent system capable of absorbing and transmitting energy originating from radioactive decay of a sample without attenuating that energy significantly. The solvent also contains a scintillator material and the combination of the sample, the solvent and the scintillator is commonly called a "scintillator cocktail." When radioactive decay occurs, it is followed by energy transfer from the radionuclide through the solvent system to the scintillator material in such a way that the scintillator material is activated and scintillates, thereby releasing light photons.
Early liquid scintillation analyzers were based on solitary photomultipliers for multiplying light signals resulting from scintillations within the cocktail with the photomultiplier output being amplified and analyzed by a suitable pulse analyzer. Such analyzers were susceptible to a variety of problems arising from inherent system noise as well as interfering radiation from external and internal sources leading to inaccurate measurements. This problem was solved t a certain extent by the advent of coincident pulse detecting liquid scintillation measurement systems which utilize a pair of photomultipliers aligned along a common axis with the radioactive sample being placed in between the two photomultipliers An electrical pulse arising as a result of the presence of radioactive emission from within the sample is considered to be the result of a legitimate radioactive event only if corresponding pulses from each of the photomultiplier tubes arrive at a coincidence detector within a predefined resolving time interval. By requiring the coincidental registration of radioactive events such liquid scintillation analyzers provide reasonable accuracy of measurement by minimizing the degrading effects arising due to the internal noise generated from within the system.
Such conventional measurement systems provide a certain degree of discrimination against some background events, particularly those due to internal system noise. However, they are critically affected by the existence of background radiation activity which may be generated by a variety of factors including Cerenkov radiation, environmental radiation, cosmic rays, static electricity, chemiluminescence, residual radioactivity of the vial or container for the scintillation cocktail and the glass used in the photomultiplier tubes and other proximate components. Radiation from these sources produces scintillations within the scintillation cocktail, sample vial or photomultiplier tube glass which become confused with, and are recorded as, valid cocktail scintillations resulting from actual radioactive decay of the sample. Protection against the effects of external radiation on such measurement systems is accomplished by the provision of a large mass of shielding material, usually lead, to form a radiation guard surrounding the liquid sample. This passive shielding, however, is not totally perfect in preventing external radiation from penetrating the system and may contain its own interfering radiation in the form of contaminants. Additionally, lead shielding is ineffective against contaminating radiation in components that are contained within the shield, such as the sample vial and the photomultiplier tubes.
Additional reduction of the effects of background radiation in liquid scintillation measurements has been accomplished by the use of guard systems employing scintillation materials, additional photodetectors sensing scintillations within the guard scintillator and a set of electronic circuits operating in anti-coincidence with the sample measurement circuitry.
Such systems, although very good at sensing and eliminating erroneous background radiation, are extremely expensive, bulky and inconvenient. As with lead shielding, they are only partially effective in eliminating the effects of background radiation from contaminants within the shielded area.
Subsequent improvements in liquid scintillation measurements have involved the use of a solid scintillation material as a guard and anti-coincidence arrangements with the coincident pulse detectors that are used by the system to detect sample scintillations also being used to detect scintillations in the guard scintillator. These systems include timing discrimination means capable of distinguishing between the fast decay times of scintillations from the liquid scintillatrr caused by radioactive events from within the test samples and those scintillations which are induced in the guard scintillator as a result of background radioactivity and have slower decay times. Although the accuracy of measurement is improved, such measuring systems are still susceptible to the effects of background radioactivity, particularly in the measurement of radioactivity levels of materials, such as tritium, which have relatively low energy emission levels, making them less easily distinguishable from the various background radiation sources.
The parent application Ser. No. 06/721,266, now U.S. Pat. No. 4,651,006, discloses a system which provides improved discrimination between background pulses and those pulses generated by true radioactive events in the sample, thereby improving the efficiency and accuracy of measurement by the use of liquid scintillation analyzers. The system utilizes certain of the inherent characteristics of the pulses being detected in order to ascertain whether they are valid sample pulses generated by sample optical events which are pulse-producing events resulting from the radioactive decay of the sample to be measured or invalid background pulses which are pulses generated by anything other than a sample optical event. This includes background optical events and electrical noise. The main sources of background optical events are scintillations of the sample vial or a photomultiplier tube glass caused by internal contaminating radiation contained within the detection system and its associated shield materials, as well as external radiation that has penetrated the system shield. The background discrimination technique is based on the premise that most background pulses triggering the coincidence detectors of the pulse detection systems have a series of randomly spaced pulses (a characteristic pulse burst) of relatively smaller energy levels and of approximate single photoelectron amplitude immediately following a coincident pulse.
In the system described in the aforementioned parent application, a particular coincident pulse is assumed to have been generated as a result of background scintillation and is disregarded in the determination of the overall energy spectrum of the radioactive sample being tested, if tracking of the output of the photomultiplier tubes for a predetermined period of time after the detection of the coincident pulse reveals that the characteristic burst of low energy pulses has occurred. The number of pulses detected in the pulse burst (the burst count) following a coincident pulse within a predetermined time period (approximately 5 microseconds) is utilized directly to determine whether or not the coincidence pulse is to be counted. Alternatively, the burst count is used in conjunction with the energy level of the coincident pulse to calculate the probability that the detected coincident pulse is the result of a valid sample scintillation.
The above type of burst detection technique provides more accurate measurements as compared to conventional liquid scintillation systems by generally improving discrimination between background and valid sample pulses. However, if the number of pulses existing in a characteristic pulse burst produced by an invalid background event is very small, the threshold number of pulses that must be detected within the predetermined time period following a coincident pulse in order to signify an invalid optical event is correspondingly small. In cases where the threshold number is extremely small, distinguishing between valid and invalid optical events is difficult because of the after-pulsing phenomenon in the photomultiplier tubes, which sometimes produces a small number of pulses following a valid optical event. There is, hence, an increased probability of misclassifying valid and invalid optical events.
This problem is of particular significance when the radionuclide under test has a low level of radioactivity so that the number of radioactive disintegrations and hence the valid count rate resulting from sample radioactivity is low enough to be comparable with the count-rate resulting from invalid background events. In such cases, even a few misclassifications can critically affect the sensitivity of measurement by significantly lowering the figure of merit for the measurement system, thereby increasing the probability that a background event will be falsely treated as a valid optical event.
The present invention ss aimed at solving this problem by adapting the burst-detection technique of the parent application in a manner that enhances the ability of liquid scintillation analyzers to accurately discriminate between pulses generated due to valid radioactive events and those generated by background radiation activity, especially in the case of extremely low-level radionuclides.